Though the present invention is described herein particularly as it relates to the handling of a finely pulverized solid material, polyvinyl alcohol (PVA), which is widely used at this time as a binder during the paper manufacturing process, to enable ready understanding of the invention it is not intended to restrict the present invention to methods and apparatus for handling PVA. It is within the spirit and scope of the invention to provide for handling and conveying of a wide variety of pulverulent materials of flour-like or dust-like consistency, to insure that environmental air in the immediate vicinity of the pulverulent material handling system remains substantially free of contact with the pulverulent material being handled and are also protected from potentially breathing air that might be laden with the fine particulate.
In the manufacture of paper a binder agent in the form of a finely comminuted solid material is typically mixed with a carrier liquid, typically water, so that the paper fibers can be bound in sheet form when screened, rolled and dried. In the past various binder agents such as corn starch have been comminuted to a fine powder-like or flour-like form and mixed with a carrier liquid to form a binding mixture. Paper fiber is then introduced to the binder mixture and is then subjected to screening to define sheet paper that is subjected to rolling and drying to complete the paper manufacturing process. Of late it has been found that polyvinly alcohol (PVA) functions as a paper fiber binder of exceptional quality for the manufacture of paper and various paper-like products. Typically powdered PVA is brought to the general site of its use in small transport bags capable of handling 50 pounds or so of the flour-like pulverulent material. Typically, the bagged material is transported to the mixing site and the bags are manually cut and the material is dumped directly into a typically large mixing vessel containing a quantity of carrier liquid. After thorough mixing by stirring the mixture is pumped or otherwise conveyed to a typically adjacent site where the paper fiber is mixed with it or it is mixed with paper fiber. It is well known that when the fine powder-like binder is poured from the bags into the liquid carrier a quantity of the fine particulate does not enter the liquid, but rather becomes entrained in the environmental air of the building housing the mixing process because it must pass through the air to the liquid. Alternatively, in the past the binder material has been unloaded from bags in its dry state and conveyed to a mixing site. In this case the pulverulent material is unloaded from bags into a hopper and is then conveyed in dry pulverulent form to a mixing facility located at the site of its intended use, where it is then conveyed into a mixing vessel and mixed with a liquid carrier agent. The liquid binder and carrier mixture is then conveyed from the mixing site to a paper manufacturing process in liquid form where the paper fibers are mixed with it.
It has been determined that handling of the dry, finely comminuted particulate during its dumping from bags and during its mechanical conveying to a site for mixing can create hazards to workers. If the flour-like particulate should constitute a health hazard to workers, if breathed, or if skin contact occurs, the workers must then be provided with appropriate equipment to ensure their isolation from the material. Such is, of course, an undesirable condition, not only because of the potential hazards to workers but also since the cost of the process is materially increased by the requirement for expensive protective equipment for the workers. In some cases a finely comminuted pulverulent material, though not an any manner hazardous when contained in bags, nevertheless can become quite dangerous from the standpoint of explosion when it is entrained in the environmental air of a closed manufacturing facility. Coal mine explosions are a well known example of the explosive nature of coal dust in the closed environments of underground coal mines. It is desirable therefore to provide for unloading and conveying of finely comminuted pulverulent materials in a manner that minimizes the potential for entrainment of the material in the environmental air of a mixing facility and therefore does not constitute a hazard to workers and does not add materially to the cost of the process.
Since PVA, being a rather dense solid, has a tendency to separate from its solution with water, it is highly desirable to maintain the water/PVA mixture in a state of continuous movement or circulation to maintain the pulverulent material in efficient solution and therefore maintain the desirable properties of the mixture until its use. Further, if the process is shut down for any reason, since settling of the solid particulate will occur within a fairly short period of time, it is desirable that the efficient character of the mixture be restored prior to its use in a paper manufacturing process.
When bags of PVA are opened and poured into a water containing vessel in a conventional manner a minute portion of the PVA will typically become airborne in the environmental air near where the mixing process is being conducted. When the PVA is so airborne, depending upon the quantity of pulverulent material being entrained into the environmental air, a potentially explosive air/powder mixture can result. In this regard it should be born in mind that many other pulverulent materials may also develop an explosive mixture when airborne in a closed environmental region such as the air within a processing building. It is highly desirable therefore to provide a novel system for the mixing of pulverulent PVA or other pulverulent materials with water or other carrier liquids in such manner that the pulverulent material does not become entrained in the environmental air so that an explosive condition is avoided.
It has been determined that PVA and other pulverulent materials may be delivered for mixing with water by means of large bulk bags which may contain as much as 1000 pounds or so of the pulverulent material. These large bulk bags are provided at one end with lifting loops to enable the filled bag to be lifted by an overhead crane, by a fork lift truck or by other hoist apparatus. A bulk bag that is suspended above a receiving hopper with its lower end located near the bottom of the hopper is then opened at its bottom to permit gravity feed of the pulverulent material from the bag into the hopper. Obviously, if the bag is merely opened at its lower end so that its contents can be deposited within a hopper for further processing, the environmental air can become contaminated as the pulverulent material passes through the air and into the hopper. Though such seldom occurs, a sufficient amount of the particulate may become entrained in the air to a point that an explosive mixture can result in the same manner as discussed above in connection with manual emptying of 50 pound bags of the pulverulent material into a carrier liquid containing vessel or into a material transferring or processing hopper. Thus, even in the case of large bulk bag transfer of the pulverulent material, it is desirable to provide means for preventing or significantly minimizing potential liberation of a quantity of the pulverulent material into the environmental air as the transfer process is being conducted.